Morpheme structure constraints are distributional laws governing sound combinations within morphemes. They specify which phoneme sequences can occur at the beginning, middle, or end of morphemes, and which combinations are forbidden. These constraints are language-specific but systematically exclude phonetically awkward or articulatorily difficult combinations.
From your analysis of phoneme inventories, you know that every language has a finite set of phonemes — the contrastive sound units that distinguish meaning. But knowing a language's phoneme inventory doesn't yet tell you how those phonemes can be combined. Phonotactics is the system of rules governing which sequences of phonemes are permitted — at the beginnings of syllables, within them, and at their ends. Morpheme structure constraints (MSCs) are the component of phonotactics that applies specifically to the internal structure of morphemes, the minimal meaning-bearing units.
English phonotactics permits many consonant clusters at the onset of syllables but not all possible combinations. "Street" begins with /str/, "split" begins with /spl/, and "strong" begins with /str/ — these are all legal. But no English word begins with /ng/ (the sound at the end of "ring"), even though /n/ and /g/ are both legitimate English phonemes. No native English morpheme begins with /tl/ or /dl/, though both phonemes exist and words like "atlas" include them. These gaps are not accidents — they reflect systematic constraints on how English sequences sounds at particular positions in the syllable. The constraint is positional: /ng/ can appear in a syllable coda (final position) but not an onset (initial position).
Morpheme structure constraints are language-specific, and this specificity is what makes foreign accents and loanword adaptation possible. Japanese has very restricted consonant clusters — most syllables follow a consonant-vowel pattern — so when English loanwords enter Japanese, clusters are broken up by inserting vowels: "McDonald's" becomes something like "Makudonarudo." Spanish does not permit word-initial /sp/, /st/, /sk/ clusters, which is why Spanish speakers learning English may add an /e/ before them ("I study español" → the initial vowel in "España" reflects the /esk/ adaptation). These adaptations are not errors but the application of the speaker's native phonotactic system to foreign material.
The theoretical importance of MSCs is that they reveal the underlying grammar of sound combination — a tacit knowledge every native speaker has but rarely consciously articulates. Native English speakers will immediately recognize that "blick" could be an English word even though it isn't, while "bnick" couldn't be, because "bl" is a legal English onset and "bn" isn't. This intuition — distinguishing accidental gaps (possible words that happen not to exist) from systematic gaps (impossible words given the phonotactic constraints) — is precisely what MSC analysis aims to capture. Mapping the constraint system for a language reveals how articulatory and phonological pressures have shaped what sound sequences its speakers find natural or foreign.